PHYSICAL REVIEW APPLIED 5, 044019 (2016)
Quench-Induced Degradation of the Quality Factor in Superconducting Resonators M. Checchin* and M. Martinello Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA and Department of Physics, Illinois Institute of Technology, Chicago, Illinois 60616, USA
A. Romanenko, A. Grassellino, D. A. Sergatskov, S. Posen, and O. Melnychuk Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
J. F. Zasadzinski Department of Physics, Illinois Institute of Technology, Chicago, Illinois 60616, USA (Received 13 November 2015; revised manuscript received 7 March 2016; published 28 April 2016) Quench of superconducting radio-frequency cavities frequently leads to the lowered quality factor Q0, which had been attributed to the additional trapped magnetic flux. Here we demonstrate that the origin of this magnetic flux is purely extrinsic to the cavity by showing no extra dissipation (unchanged Q0 ) after quenching in zero magnetic field, which allows us to rule out intrinsic mechanisms of flux trapping such as generation of thermal currents or trapping of the rf field. We also show the clear relation of dissipation introduced by quenching to the orientation of the applied magnetic field and the possibility to fully recover the quality factor by requenching in the compensated field. We discover that for larger values of the ambient field, the Q-factor degradation may become irreversible by this technique, likely due to the outward flux migration beyond the normal zone opening during quench. Our findings are of special practical importance for accelerators based on low- and medium-β accelerating structures residing close to focusing magnets, as well as for all high-Q cavity-based accelerators. DOI: 10.1103/PhysRevApplied.5.044019
I. INTRODUCTION Superconducting radio-frequency (SRF) cavities are resonant structures that allow accelerating charged particles up to energies of tera-electron-volts [1–3]. The limiting factors of such accelerating structures are represented by the finite value of the intrinsic quality factor Q0, directly related to the cryogenic cost needed for their operation, and by the radio-frequency- (rf-)field breakdown due to quench that limits the maximum achievable accelerating gradient Eacc . A typical quench event is initiated by a small area of the cavity surface becoming normal-conducting either due to heating up above the critical temperature (T c ) or due to the local critical field being exceeded. The sharp increase of the surface resistance in the normal zone can be contained only up to a certain dissipation level, above which a fast avalanchelike spreading of the normal zone that consumes all of the rf field in the cavity occurs. Several known mechanisms [1,2,4–6] may cause quench, and it was hypothesized that when the normal-conducting region is *
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created, some magnetic flux can be trapped at the quench spot causing extra dissipation [7]. The origin of such trapped magnetic flux remained unclear and was ascribed to different mechanisms, such as thermal currents driven by the local thermal gradient in the quench zone [7], rf field trapped within the penetration depth region, or ambient magnetic field. However, a full understanding of the phenomenon has not been developed yet. Previous studies [8–11] of the quality factor degradation in high- and medium-β superconducting resonators targeted a criterion for the amount of flux trapped during the quench. A clear dependence of the quench-related degradation on the locally applied nonuniform external magnetic field was found, highlighting the possibility that extra dissipation introduced by quenching was of environmental origin. The “quench annealing” phenomenon—the recovery of the cavity quality factor by quenching when the additional field was removed—was also documented in these studies. Some deficiencies of these studies were the lack of advanced thermometry mapping and nonuniform magnetic field environment, making a full description challenging. Also, higher than typical magnetic field values were explored, similar to those found in special cases such as cavities operating close to a strong magnet. In the presented study, we use the full range of current state-of-the-art techniques including advanced temperature
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M. CHECCHIN et al.
PHYS. REV. APPLIED 5, 044019 (2016) in detail the resulting dissipation pattern, the one-cell cavities are equipped with the advanced temperaturemapping system (T map) [14] based on an array of carbon-resistive sensors placed on a total of 36 boards— 16 per board—with the boards positioned every 10° around the cavity circumference. The external magnetic field is sustained by Helmholtz coils and measured by four singleaxis Bartington Mag-01H cryogenic fluxgate magnetometers positioned at the equator axially to the cavity and spaced about 90° between each other; see Fig. 1(a) for the schematic. For one of the cavities (AES011), two sets of Helmholtz coils are used generating fields in two different directions (axial and orthogonal). In this configuration, no temperature mapping is used due to space constraints. As shown in Fig. 1(b), the fully dressed LCLS-II nine-cell cavity (AES024) is equipped with two sets of Helmholtz coils and three fluxgate magnetometers placed outside of the helium vessel. 150
Coil T map 114
Fluxgate
150
mapping and Helmholtz coils to understand the detailed physics behind the quality factor degradation due to quench in superconducting resonators. In SRF applications, our results can be helpful for Q preservation in accelerators utilizing cavities at very high-Q values (requiring very challenging magnetic field shielding and cooldown process), as well as for designing cryomodules where SRF structures need to operate nearby sources of high magnetic field (usually solenoids or quadrupole magnets). We report the experimental proof that the Q0 degradation due to quench is a direct consequence of trapped ambient magnetic field, ruling out any other possible mechanisms. We also demonstrate that a full recovery of Q0 after quench can be achieved when the cavity is quenched in the absence of the external magnetic field—an alternative to warming up above the critical temperature—and present a consistent physical model of this phenomenon. In addition, we find a dependence of the extra losses after quench on the orientation of the external magnetic field with respect to the cavity axis. To understand the recovery of Q0 , the key is the configuration of the magnetic field trapped at the quench spot, which we discuss in detail. We observe that the recovery of the quality factor is not possible if the externally applied field is big enough (>1 Oe). The proposed explanation for this irreversibility is the migration of the flux farther from the quench spot. II. EXPERIMENTAL SETUP Quench experiments are performed using multiple 1.3-GHz fine-grain bulk Nb cavities of elliptical TESLA shape [12]. Three bare one-cell cavities and one dressed nine-cell cavity are prepared by nitrogen-doping recipes, and one one-cell cavity is prepared by a standard EP þ 120 °C baking international linear collider (ILC)-type recipe (Table I). Nitrogen-doped cavities (nine-cell cavity included) are baked for 3 h at 800 °C before the doping treatment. All measurements are done at the FNAL cavity vertical test facility. Schematics of the cavity instrumentation used are presented in Fig. 1. In order to map the temperature variation over the cavity wall during quench, localize the quench spot site, and study
106.3
(a)
Cavity AES011 AES019 ACC002 AES014 AES024
280
TABLE I. Cavities studied with respective thermal treatments and quench fields. Doped cavities are treated with 25 mTorr of N2 and with a post-treatment chemistry (EP) of 5 μm.
1026
~ 400
280
280
Processing treatment
Cavity type
800 °C, 2 min with N2 þ 6 min without N2 800 °C, 10 min with N2 800 °C, 20 min with N2 120 °C bake 800 °C, LCLS-II N-doping treatment [13]
Bare one cell Bare one cell Bare one cell Bare one cell Dressed nine cell
(b) FIG. 1. Experimental setup for (a) one-cell cavities and (b) a nine-cell fully dressed LCLS-II cavity. All the dimensions are given in millimeters.
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QUENCH-INDUCED DEGRADATION OF THE QUALITY … In order to minimize the temperature-dependent part of the surface resistance, all the measurements except for the nine-cell cavity (measured only at 2 K) are done at the lowest temperature achievable by the cryoplant, which is around 1.5 K. III. RESULTS All the measurements are performed by quenching cavities in the presence of the external magnetic field (H) or in compensated magnetic field and by recording the degradation of Q0 at the fixed accelerating field caused by the quench. The quench events considered are caused only by “hard” limiting factors (e.g., thermal breakdown), whereas multipacting or field-emission-related quenches are not considered in this study. The very low-compensated magnetic field (
